1. Field of the Invention
The present invention relates to a method for regenerating a chromium oxide-based catalyst subsequent to its use in the production of chlorine by catalytic oxidation of hydrochloric acid.
2. Description of the Related Art
Chlorine is produced on a large scale by electrolysis of brine. The demand for chlorine has increased year by year. However, caustic soda is formed at the same time upon electrolysis of brine. Since the demand for caustic soda is lower than that for chlorine, difficulties are now increasingly recognized in the achievement of adequate balancing between the by-production of caustic soda from brine electrolysis and its consumption.
Hydrogen chloride is by-produced in a large quantity upon chlorination of organic compounds or upon reaction of organic compounds with phosgene. The by-produced quantity of hydrogen chloride is however much greater than its demand, so that a great deal of hydrogen chloride is disposed of as a waste product, yet costing substantial expenses for its disposal.
It is possible to meet the increasing demand for chlorine without developing an imbalance in demand with respect to caustic soda provided that chlorine can be efficiently reclaimed from hydrogen chloride.
Production of chlorine by oxidation of hydrogen chloride has been known under the name of the Deacon process for many years. Copper-base catalysts, the history of which goes back to the invention of Deacon in 1868 (British Patent No. 1403 granted in 1868), have conventionally been considered to exhibit the best activity. Numerous catalyst systems have been proposed to date, which contain one or more of various compounds as a third component in addition to copper chloride and potassium chloride. To oxidize hydrogen chloride at an industrially-acceptable reaction velocity by using these catalysts, it is however necessary to raise the reaction temperature to at least 450.degree. C., thereby developing another problem that the service life of the catalysts is reduced due to flying-off of the catalyst components. In addition, the oxidation reaction of hydrogen chloride is a reversible reaction, and a lower temperature hence results in a higher concentration of chlorine at equilibrium. It is therefore essential to develop a catalyst which can exhibit its activity at a temperature as low as possible.
With the foregoing in view, catalyst systems other than copper-based catalyst systems have bee investigated.
It has been proposed to use a chromium oxide-based catalyst as an oxidation catalyst for hydrogen chloride because chromium oxide has better stability and durability to heat than copper-based catalysts.
For example, a process for the production of chlorine has been proposed in British Patent No. 584,790, whose disclosure is incorporated herein by reference in which hydrogen chloride is caused to flow at 400.degree. C. over a chromium oxide catalyst prepared b impregnating a suitable carrier with chromic anhydride or with an aqueous solution of chromium nitrate and then thermally decomposing chromic acid anhydride or chromium nitrate. When the catalyst becomes deactivated, the feeding of hydrogen chloride is stopped and air is instead caused to flow to regenerate the catalyst. After the catalyst has been regenerated, the feeding of air is stopped and hydrogen chloride is again caused to flow.
A conversion of hydrogen chloride at a rate as high as 67.4% based on the theoretical value was reported in British Patent No. 676,667, whose disclosure is incorporated herein by reference, by reacting hydrogen chloride and an oxygen containing gas at a reaction temperature of 420.degree.-430.degree. C. and the space velocity of 380 hr.sup.-1 while using a catalyst with chromium oxide of a dark blackish color or a dichromate supported on a carrier. In the above reaction, the conversion was 63% based on the theoretical value when the space velocity was 680 hr.sup.-1. Although the reaction was found to proceed even at 340.degree. C., the conversion was as low as 52% based on the theoretical value even when the space velocity was set at the low value of 65 hr.sup.-1.
The above two processes both require a high reaction temperature and do not permit the use of a high space velocity, so that their practice in the industry is difficult.
It is reported in Japanese Patent Application Laid-Open No. 275104/1986 whose disclosure is incorporated herein by reference that a chromium oxide-based catalyst obtained by calcining at a temperature not higher than 800.degree. C. a compound obtained by reacting ammonia with an aqueous solution of chromic acid, shows high activity in the oxidation reaction of hydrogen chloride. The use of this catalyst has made it possible to produce chlorine at a lower temperature and with a greater space time yield compared to the catalysts conventionally known.
A problem associated with the use of this catalyst is that when it is used in the oxidation reaction of hydrogen chloride gas, its activity is reduced within several months after the initiation of the reaction. As a reactivation method therefor, it has been proposed in Japanese Patent Application Laid-Open No. 254846/1987 to bring it into contact with hydrogen chloride gas and/or an oxygen-containing gas in a high-temperature vapor phase. However, when a catalyst reactivated by this method is employed in the oxidation reaction of hydrogen chloride gas, although the catalyst can exhibit activity as high as a fresh catalyst for several days after the initiation of the reaction, its activity is reduced after one week and the catalyst thus reactivated cannot therefore be used for a long period of time.